Project Summary Toxoplasma gondii is arguably one of the most successful parasitic protozoans on the planet. It is estimated that T. gondii infects approximately a third of humanity (~2 billion people) and nearly half of all warm-blooded animals. Once established, infection is lifelong and characterized by the presence of semi-dormant tissue cysts typically in muscle and neural tissues. As host conditions change, T. gondii has the capacity to reactivate and induce multiple rounds of recurrent parasitemia, a fact which becomes an issue of serious concern in an aging population or in individuals with compromised immune systems. Currently, there are no drugs available that can effectively treat the chronic stage of infection and therapies currently in use against acute infection are often accompanied by serious side effects. It is therefore important, especially as population's age, to generate new methods and compounds to treat this disease. Upon invasion of its host cell, Toxoplasma induces a massive reorganization of the host cell transcriptional program and as a result, major pathways related to metabolism, immunity and the cell cycle become activated. T. gondii has been show to initiate modification of its host cell environment through the regulated secretion of its various secretory organelles (micronemes, rhoptries, dense granules). The dense granules, originally characterized as serving only to condition the parasite vacuole to be a feeding organelle, are now considered a reservoir of various host nuclear targeted parasite effectors with diverse targets and functions. These proposed studies aim to characterize a newly identified parasite effector of the dense granules we have termed ICC1 for inducer of cell cycle gene 1. We have demonstrated that ICC1 traffics to the host cell nucleus soon after infection and induces the expression of a multitude of genes involved in progression of the host cell cycle into S-phase. Additionally we have found that T. gondii also has 2nd redundant effector to alter the host cell cycle via that is released from its rhoptry organelle and which we have identified as the host nuclear targeted phosphatase PP2C- hn. To advance our knowledge of ICC1 and PP2C-hn's function, we propose to perform a series of immunoprecipitation and mass spectrometry experiments in order to identify the principal host targets and determine the functional significance of this interaction as it relates to cell cycle progression. The results of this proposal will provide the first mechanistic basis for the modulation of the host cell cycle by a protozoan parasite and may give additional insight into the basic function of our own cells.